This application claims priority of Great Britain Application No. 0110155.9 filed on Apr. 25, 2001.
The present invention relates to a method for overload control in a telecommunications network. The present invention also relates to apparatus therefor.
The network can be a UMTS or other third generation (3G) network.
Calling rate patterns in telecommunication networks typically have predictable daily profiles with occasional very high peaks superimposed. These peaks can be caused by events such as telephone voting due to a television program, or disasters, etc.
Although it would be possible to dimension a network to handle such peak traffic, it would be uneconomic to do so. In addition, mass call events frequently have several source nodes, but are focussed on a few, or just one, destination, and will overload it. Network dimensioning cannot solve a destination""s overload, and some form of overload control is required to manage peak traffic.
Many mass call events are unexpected, so the control needs to be automatic. Overloads often increase rapidly (within seconds in some cases), persist for minutes with large fluctuations in amplitude, and dissipate over a somewhat longer period. This means that the control needs to be fast in application, and be able to adapt readily to changing loads.
Ideally, the nodes whose callers caused the overload event should exert control, since they have the possibility of informing these callers of the situation. However, the target node for the overload needs to be able to control its input until the sources have time to react. Here, actions by the target node are referred-to as xe2x80x9clocal overload controlxe2x80x9d. The target node often does the minimum work necessary upon calls to control its local overload, in order to protect itself in the short term. Such local overload control has the danger that, if callers are not told promptly of the overload, they will reattempt their calls. This could cause the overload to spread through the network.
Hence, network signalling systems such as International Telecommunications Union ITU-T Signalling System No.7 (SS No.7) have defined as part of their protocol messages and procedures to be used by a target node to inform its adjacent source nodes when it is in overload. SS No.7 has defined an xe2x80x9cautomatic congestion controlxe2x80x9d (ACC) procedure and an xe2x80x9cautomatic congestion levelxe2x80x9d (ACL) parameter to be used in all call release messages by an overloaded node. The ACL parameter has one of two possible values xe2x80x9cless severexe2x80x9d or xe2x80x9cmore severexe2x80x9d, but the meaning of these values at the target node is not further defined. The behaviour of the target node is also not defined.
Source nodes are expected to reduce their traffic towards the overloaded node when they receive a release message containing an ACL parameter from it, and if possible they inform their local callers of the situation. Such attempted restriction of traffic towards a remote node is called xe2x80x9cremote overload controlxe2x80x9d here. The speed at which traffic can be reduced, and the degree of reduction is not defined in SS No.7, but they depend upon the traffic level, the implementation of the source and target nodes, and the network.
In order to reduce the amount of call re-attempts, the traffic offered by sources to the target node should be held close to, preferably just under, the capacity of the target node. This ensures its optimum performance during emergencies.
The work performed by source nodes in remote overload control should not cause the source itself to be come overloaded.
Many solutions for remote overload control in SS No.7 maintain an overload level indication for a remote target node, the indication depends upon the value of the ACL parameter received, and frequently upon the rate at which release messages containing ACL parameters are received from it. A timer is started at the reception of the first release message containing an ACL parameter, which is restarted if such a release message is received before the timer expires. The overload level indication is increased upon reception of a release message containing an ACL parameter, and decreased if the timer expires. When the indicated overload level reaches zero, traffic restriction is stopped.
Some solutions limit the speed at which the overload level indication can be increased, by use of a short guard timer which is started upon reception of the first release message containing an ACL parameter. Further such release messages received whilst the short timer is running are ignored in the remote overload control procedure. Receipt of a release message containing an ACL parameter when the short timer is not running, whilst the long timer is running, cause the short timer to be restarted, the long timer to be restarted, and the overload indication to be updated.
Some solutions for remote overload control use a form of proportional reduction of traffic offered to the sources for the target node, the proportion depending upon the indicated overload level. Other solutions use some form of rate limiting mechanism, such as a leaky bucket, the rate of calls offered by the source node to the target depending upon the indicated overload level. Rate limiting schemes are better than proportional discard schemes in that they limit absolutely the traffic offered to the target node.
It is known from U.S. Pat. No. 5,450,483 to provide a rate limiting scheme using a leaky bucket in source nodes, whose rate is set by another leaky bucket there recording the rate at which call rejects are received from the overloaded target node. This requires the target to be kept just over its overload level. It uses the ISDN User Part ACC mechanism (described in ITU-T Recommendation Q.764 (09/97) section 2.11).
It is known from International patent application WO99/38341 to provide a scheme which uses a continuous xe2x80x9coverload control levelxe2x80x9d (OCL), whose value is set by receipt of call rejects as in Q.764 section 2.11, against calls released normally. The OCL value determines what proportion of calls offered to the source for the target will be discarded. It is not an absolute rate limiting scheme.
Each known remote overload control method has its own limitations. Methods using an overload indication level at the source node tend to impose an initial traffic restriction at the minimum discrete level, the restriction increases with further overload indications. If the onset of overload is rapid, and the offered traffic is high, the initial restriction level tends to be so low that the target is pushed into an overload state from which it can recover only by extreme measures such as message discard.
With a discrete number of overload levels, in order to get a smooth response the number of levels tends to be high. This causes a slow response to sudden overload. In addition, the response by the source to removal of the overload tends to be slow, and the traffic offered to the target by the source tends to drop significantly there.
If the restriction method is to discard a proportion of the offered traffic according to the overload indication, a severe traffic overload offered to the source can result in a severe traffic overload offered by the source to the target.
The method described in U.S. Pat. No. 5,450,483 mentioned above requires the target node to be kept just over its overload level. If the target node employs any technique such as discard of some call set up messages, or delays rejection of them to reduce the possibility of reattempt by the caller, the apparent overload level recorded need not be the actual overload level, and the behaviour of the network might be different from that expected. In addition, the efficiency of the target is reduced by its need to reject calls.
The method described in WO99/38341 mentioned above requires a mix of release messages with and without ACL parameters to fix the OCL value. It uses a proportional discard scheme, and hence is liable to be slow in reacting to a rapid overload. In addition, some types of call (e.g. those which originate in the public switched telephone network and destined to the Internet) are cleared down in normal circumstances only by the caller. Consequently, a switch used to connect to the Internet will clear down calls only when overloaded, and so would normally not send a release message without an ACL parameter. Thus a source local exchange using this method for remote overload control sending calls to a once overloaded Internet-connected switch will assume it is still overloaded.
Other known methods have a serious weakness, which is caused by use of a timer to reduce restriction to the target node. Overloads can be of any duration, if expiry of a timer causes the restriction to be lifted or even just decreased at the source, with the traffic offered to the source for the target still greater than the target""s capacity, the target will enter overload again. During long duration periods of high offered traffic, the target thus oscillates into and out of overload.
The present invention provides a method of suppressing overload of a first node in a telecommunications network by the steps of a second node: receiving a signal indicating that a call directed from the second node to the first node has bee rejected and in response restricting the rate of transmission of calls from the second nod to the first node to a predetermined level, and monitoring duration through a predetermined time period wherein if a further signal indicating that a call directed from the second node to the first node has been rejected is received during the predetermined time period, the rate of transmission of calls from the second node to the first is restricted further to a further reduced predetermined level and duration through a further predetermined period is monitored.
The preferred methods react quickly to a sudden onset of overload, they both keep the target just under overload during the duration of high offered traffic at the source. They do not allow the target to oscillate into and out of overload. Variations of load offered to the source node for the target, above the target""s capacity, are filtered out by the source node.
Preferably the or each predetermined level of restriction is selected dependent upon the measured rate of calls (Dmeasured) from the second node accepted by the first node.
As the level of restriction (xe2x80x9cleaky bucketxe2x80x9d rate) is determined by the measured capacity of the target node, any changes to that capacity cause the leaky bucket rate to change. Thus modifications to the software or hardware at the target node which cause its capacity to change are automatically cateredxe2x80x94for, they do not need management intervention at the source. Management intervention at the source is also not required if there is a change in the number of other sources in the network.
Preferably the steps of restricting and monitoring are repeated until no signal indicating that a call directed from the second node to the first node has been rejected is received during the latest predetermined period.
The source nodes do not need to communicate with each other in order to jointly control overload at a target node.
The present invention also provides a telecommunications network comprising a first node and a second node, the second node comprising receiving means operative to receive a signal indicating that a call directed from the second node to the first node has been rejected, and restricting means operative in response to both restrict the rate of transmission of calls from the second node to the first node to a predetermined level and start its timer to monitor duration through a predetermined period, wherein in use, if a further signal indicating that a call directed from the second node has been ejected is received by the receiving means during the predetermined period being measured by the timer, the restricting means reduces the rate of transmission of calls from the second node to the first node to a further predetermined level and restarts the timer.
The present invention also provides a telecommunications apparatus comprising receiving means operative to receive a signal indicating that a call sent from said apparatus has been rejected, the apparatus comprising restricting means operative in response to restrict the rate of calls sent out and start its timer to monitor duration through a predetermined period, wherein in use, if a further signal indicating that a call sent from said apparatus has been rejected is received by said receiving means during the predetermined period measured by the timer, the restricting means further reduces the rate of calls sent out.
The preferred methods are not limited to remote overload control by one node of one other in a telecommunications network. Each source node can monitor all target nodes with which it was concerned, so that if any overload of any target occurs, the associated source nodes can then perform the corrective action of automatic congestion control.